testgles2.c: refresh cached SDL_GL_GetDrawableSize() on SDL_WINDOWEVENT_SIZE_CHANGED, not SDL_WINDOWEVENT_RESIZED

Fixes bug with viewport not updating when moving window between monitors with different scale
factors on Windows (this should also fix the same issue on other OS'es, though untested)

test/testgles2.c

@@ -454,7 +454,7 @@ void loop()
         switch (event.type) {
         case SDL_WINDOWEVENT:
             switch (event.window.event) {
-                case SDL_WINDOWEVENT_RESIZED:
+                case SDL_WINDOWEVENT_SIZE_CHANGED:
                     for (i = 0; i < state->num_windows; ++i) {
                         if (event.window.windowID == SDL_GetWindowID(state->windows[i])) {
                             int w, h;